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Variable Interest Entities (Details) $ in Millions	3 Months Ended	6 Months Ended
Variable Interest Entities (Details) $ in Millions	Mar. 31, 2016 USD ($)	Jun. 30, 2016 USD ($) MW	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Variable interest entities
Equity investment in ATC		$ 1,425.0		$ 1,380.9
ATC
Variable interest entities
Ownership interest in ATC (as a percent)		60.00%
Equity investment in ATC		$ 1,425.0		1,380.9
Accounts payable due to ATC		$ 30.3		$ 28.3
Purchased power agreement
Variable interest entities
Firm capacity from purchased power agreement (in megawatts) \| MW		236
Minimum energy requirements over remaining term of purchased power agreement (in megawatts) \| MW		0
Remaining term of purchased power agreement (in years)		6 years
Residual guarantee associated with purchased power agreement		$ 0.0
Required payments over remaining term of purchased power agreement		107.9
Total capacity and lease payments		$ 26.9	$ 26.8
Accounting Standards Update 2015-02
Variable interest entities
Changes to disclosures and financial statement presentation	$ 0.0